The present invention is generally directed to a balanced momentum probe holder for use in metrology systems, especially scanning probe microscopes used to measure sample surfaces down to the nanometer level. Specifically, the invention is directed to such systems employing nested-Z and non-nested parallel feedback loops, to achieve rapid, and highly accurate scanning of a sample surface. The invention also relates to methods of using such a probe holder in such systems.
The ongoing miniaturization of components of a variety of devices makes high-resolution characterization of critical surfaces increasingly important. In the field of metrology, for example, surface-characterization devices such as stylus profilers and scanning probe microscopes (SPM) are routinely used to measure topography and other characteristics of critical samples. Stylus profilers and scanning probe microscopes are in fact frequently used as inspection tools to measure the critical surfaces of industrial devices like semiconductor chips and data storage devices during and after the manufacturing process. To be economically feasible, these profilers and scanning probe microscopes must complete their measurements as quickly, accurately, repeatably and as reliably as possible. The accuracy, precision, reproducibility, and reliability of such metrology instruments are especially critical in view of the ongoing desire that such surface-characterization instruments be capable of quickly and accurately characterizing dimensions smaller than those of the products and devices being fabricated, to assure manufacturing quality, and to provide accurate diagnoses of manufacturing problems. Because critical features continue to shrink in the manufacturing process, it is necessary to improve the accuracy and the speed of scanning probe microscopes and stylus profilers to keep up with the measurement demand.
For the sake of convenience, the discussion that follows and throughout this patent specification will focus on Atomic Force Microscopes (AFMs). In this regard, it shall be understood that problems addressed and solutions presented by the present invention shall also be applicable to problems experienced by other measurement instruments including surface-modification instruments and micro-actuated devices.
The typical AFM includes a probe which includes a flexible cantilever and a stylus mounted on the free end of the cantilever. The probe is mounted on a scanning stage that is typically mounted on a common support structure with the sample. A typical scanning stage may include an XY actuator assembly and a Z actuator, wherein xe2x80x9cXxe2x80x9d and xe2x80x9cYxe2x80x9d represent what is typically the horizontal XY plane, and xe2x80x9cZxe2x80x9d represents the vertical direction. xe2x80x9cXxe2x80x9d and xe2x80x9cYxe2x80x9d and xe2x80x9cZxe2x80x9d are mutually orthogonal directions. The XY actuator assembly drives the probe to move in an X-Y plane for scanning. The typical Z actuator, mounted on the XY actuator and providing support for the probe, thus drives the probe to move along a Z axis which is disposed orthogonally relative to the X-Y plane. (The definition of the XYZ axes is convenient and typical, but the choice of axis name and orientation is of course arbitrary.)
AFMs can be operated in different sample-characterization modes including contact-mode and Tapping(trademark) mode. In contact-mode, the cantilever stylus is placed in contact with the sample surface, cantilever deflection is monitored as the stylus is scanned over the sample surface, and the resulting image is a topographical map of the surface of the sample. In Tapping(trademark) mode (a trademark of Veeco Instruments, Inc.) sample characterization, the cantilever is oscillated mechanically at or near its resonant frequency so the stylus repeatedly taps the sample surface or otherwise interacts with the sample. See, e.g., U.S. Pat. Nos. 5,266,801; 5,412,980; and 5,519,212 to Elings et al., which are illustrative.
In either sample-characterization mode, the interaction between the stylus and the sample surface induces a discernable effect on a probe-based operational parameter, such as the cantilever deflection oscillation amplitude, the phase or the frequency, all of which are detectable by a sensor. In this regard, the resultant sensor-generated signal is used as a feedback control signal for the Z actuator to maintain a designated probe operational parameter constant.
In contact-mode, the designated parameter may be cantilever deflection. In Tapping(trademark) mode, the designated parameter may be oscillation amplitude, phase or frequency. The feedback signal also provides a measurement of the surface characteristic of interest. For example, in Tapping(trademark) mode, the feedback signal may be used to maintain the amplitude of cantilever oscillation constant to measure the height of the sample surface or other sample characteristics.
In analyzing biological samples, polymers, photoresist, metals and insulators, thin films, silicon wafer surfaces, and other surfaces, the ability to accurately characterize a sample surface is often limited by the present ability of an AFM to move the stylus vertically relative to the surface at a rate sufficient to accurately measure the surface while scanning in either the X or Y direction. This ability is inadequate in present day devices for essentially two reasons.
In order to accurately measure the height of all features, both large and small, on a sample surface, the Z actuator must have the ability to displace the stylus connected thereto over a large range of heights, i.e., it must have large vertical travel. This necessitates that the Z actuator, whether it is a scanning tube such as is on this assignee""s Dimension series AFM heads or is a flexure such as is on this assignee""s Metrology series AFM heads, must be large enough to move the stylus up and down sufficiently to measure even the largest surface features.
Unfortunately, a necessary by-product of a larger Z actuator having greater range is associated greater mass which makes the actuator movement relatively slow. Slow actuators are not able to move the probe rapidly enough in Z while scanning in X or Y at anything more than modest speed without damaging the probe or sample or without sacrificing measurement accuracy. Because it is important while scanning to minimize the force of the stylus on the sample to prevent damage to the stylus and/or sample, the scan rate in X or Y must, of necessity, be reduced to a speed compatible with the Z actuator""s ability to move the stylus up and over surface features without slamming into them, which is obviously undesirable. One present day technique to overcome this limitation and increase responsiveness of the Z-actuator is to increase the gain of its feedback loop. This works only to a limited degree because if the gain is increased more than a modest amount, the Z actuator begins to resonate and that resonance is passed into the AFM, creating parasitic oscillations, which in turn ruin image quality. In essence, a large mass, large displacement Z actuator cannot be made to overcome its inherent physical limitations.
In another approach, one does not attempt to wring more performance from the large Z actuator than it is inherently able to deliver. Instead, a separate xe2x80x9cfastxe2x80x9d Z actuator is used, with its own feedback loop, to move the stylus quickly over small surface variations that the large Z actuator is too slow to react to, which enables one to obtain relatively high quality imaging at even high scan speeds. The fast Z actuator is smaller than and hence of significantly smaller mass than the slow Z actuator. As a result it is advantageously driven in its own (or shared) fast feedback loop at speeds exceeding that of the slow Z actuator.
Unfortunately, at high gain, the high speed of operation and momentum of the fast Z actuator can similarly cause parasitic oscillations which reduce image quality. A device and method which balances these inertial forces created by a fast Z actuator would be of great benefit and commercial interest.
It is an object of the present invention to provide a novel balanced momentum probe holder for scanning probe microscopes and/or stylus profilers that permits the probe to measure the height of small surface features better than is presently possible with commercially available tools. It is specifically an object to provide such a probe holder for an improved atomic force microscope (AFM).
Another object of the present invention is to provide a novel AFM that permits more accurate imaging of surface features at high scan rates.
Still another object of this invention is to provide an AFM that can measure surface features at high scan rates without inducing parasitic oscillations in the AFM.
A further object of this invention is to balance the momentum created by the fast Z actuator in an AFM to allow fast actuation without driving parasitic oscillations.
Yet another object of this invention is to provide a fast actuator of sufficiently low mass to allow its use on the lower end of a scanning stylist AFM.
Yet a further object of this invention is to provide an AFM with fast actuation optimized for operation in nested or parallel feedback loops.
These and other objects are achieved according to the present invention by providing a new and improved AFM having a probe holder that includes a separate, fast Z actuator assembly operated in a fast feedback loop and that balances the momentum of the fast Z actuator assembly. The basic idea is to balance the momentum of the moving probe holder with the momentum of a counterbalance moving in synchronization with the probe holder, but in the opposite direction. In this case, the net momentum of the fast Z-actuator assembly is essentially zero, and thus the motion of the probe does not substantially excite parasitic resonances of the supporting structure and/or XYZ scan assembly. The fast Z actuator assembly is also of low mass and is therefore able to displace the probe in the Z direction more rapidly than a larger, higher mass conventional Z actuator which is part of the piezo tube or the flexure upon which the fast Z actuator assembly is mounted. In order to take advantage of the small size and low mass of the fast Z actuator assembly, it is operated in a fast feedback loop, either nested with the feedback loop of the conventional Z actuator or in a parallel feedback loop. The combination of a low mass fast Z-actuator and the balanced momentum enables extremely accurate scanning of even the smallest surface features and even at high scan speeds where conventional Z actuators perform sluggishly.
The present invention, then, is generally directed to an apparatus having a probe for characterizing a surface of a sample. The apparatus may have an X actuator, a Y actuator and a first Z actuator as in an AFM but may also have only a Z actuator such as in a profilometer. The apparatus also has a second Z actuator assembly with the probe mounted on it. The second Z actuator assembly is coupled to the first Z actuator. The second Z actuator assembly is less massive and therefore quicker responding than the first Z actuator. When actuated to move the probe, the momentum of the second Z actuator assembly is balanced so that its motion does not transmit substantial vibration to other actuators or support members.
The fast Z actuator assembly comprises first and second fast Z-actuators, sometimes referred to herein as the bottom actuator and the top actuator, respectively. The two actuators are arranged so that the fixed ends are attached to a common central support. Then the top end of the top actuator and the bottom end of the bottom actuator are both free to move. The measurement probe, for example an AFM cantilever probe, is attached directly or through intermediate mounting to the bottom or distal end of the bottom actuator which is proximate the sample. A counterbalance mass is attached to the top or distal end of the top actuator. The top and bottom fast Z-actuators are arranged so that they move in a synchronized manner, but in opposite directions. The probe mount, actuators, and counterbalance mass are arranged to match the momentum carried by the top and bottom actuators. In the simplest case the mass of the top actuator is the same as the mass of the bottom actuator and the mass of the counterbalance mass matches the mass of the probe mount. Then the two actuators are arranged to move substantially the same distance (in opposite directions) at the same time. Since the motions are the same but opposite and the masses are matched, the net momentum is essentially zero, thus transmitting no vibration to surrounding members. In more complicated arrangements, the momentum can be matched by arranging a top actuator with say half the motion of the bottom actuator, but twice the moving mass, or suitable variations thereof that match combinations of velocity and mass of the top and bottom fast Z-actuators.
In one embodiment, the first, bottom actuator includes a first piezo stack disposed between the common central support and the probe mount assembly, and the second, top actuator includes a second piezo stack disposed between the counterbalance and the common central support.
In yet another embodiment, the balanced momentum probe holder is incorporated into a nested feedback control system. In still another embodiment, the balanced momentum probe holder is incorporated into a non-nested parallel feedback control system.
In both feedback systems, when an error signal to move the probe vertically is sent to the fast Z actuator assembly, the first piezo stack extends or retracts to move the probe to the desired height while, simultaneously, the second piezo stack extends or retracts also. The momentum of the second piezo stack and its associated components balances the momentum of the first piezo stack and associated components including the probe.
As a result, the net momentum, and therefore the net force acting upon the larger system is eliminated, thereby eliminating or substantially reducing parasitic oscillations. In a nested feedback control system, the error signal is processed by a control device such as a PID controller and sent to the fast Z actuator assembly to cause it to move the probe. Any residual error signal is sent to the slow Z actuator assembly to cause it to move the probe an additional amount needed. In this way, the probe is able to track, and therefore measure the height of surface features that are quite small, even at high scan speeds, while also being able to measure larger surface features as well.